fix ENOBUFS errors with backoff+retry

pixelflut.c

@@ -216,18 +216,30 @@ static void *worker_main(void *arg)
 		while (next < PACKET_COUNT) {
 			int sent = sendmmsg(fd, &messages[next],
 			                    (unsigned int)(PACKET_COUNT - next), 0);
-			if (sent == -1) {
-				if (errno == EINTR)
+			if (sent >= 0) {
+				next += sent;
 				continue;
-				/* ECONNREFUSED is stale ICMP feedback on a connected
-				 * UDP socket; EAGAIN means the send buffer is full.
-				 * Both are transient - repaint and keep sending. */
-				if (errno != ECONNREFUSED && errno != EAGAIN &&
-				    errno != EWOULDBLOCK)
+			}
+			switch (errno) {
+			case EINTR:
+				continue; /* interrupted before sending - retry now */
+			case ENOBUFS: /* interface send queue full: we outrun the link */
+			case EAGAIN:
+#if EWOULDBLOCK != EAGAIN
+			case EWOULDBLOCK:
+#endif
+				/* Transient back-pressure. Back off briefly so the queue
+				 * drains (rather than spinning on failing syscalls), then
+				 * resend the same batch - no need to repaint. */
+				usleep(200);
+				continue;
+			case ECONNREFUSED:
+				break; /* stale ICMP on connected UDP; repaint and retry */
+			default:
 				perror("sendmmsg");
 				break;
 			}
-			next += sent;
+			break; /* ECONNREFUSED / unexpected: stop, repaint, retry */
 		}
 	}
 

pixelflut.h

@@ -26,7 +26,7 @@
  * to avoid IP fragmentation (e.g. 207 pixels ~= 1451 B fits a 1500 B link).
  * Override at build time with -DPIXELS_PER_PACKET=N. */
 #ifndef PIXELS_PER_PACKET
-#  define PIXELS_PER_PACKET 120
+#  define PIXELS_PER_PACKET 200
 #endif
 
 #pragma pack(push, 1)